Abstract: One-step Arrhenius reaction model was used with no consideration of the effect of viscosity, heat transmission and diffusion on flow for mixtures of H₂/Air to simulate two-dimensional rotating detonation engine(RDE) to research on RDE's effective diameter range. The inviscid flux was discreted with fifth-order WENO scheme, and the time integration was performed with three-order Runge-Kutta Scheme. The results show that the critical diameter of RDE is 0.51l in specific conditions, where l refers to the axial length, only when the diameter is larger than the critical diameter can RDE work properly. As the diameter increases, the thrust, specific impulse, detonation wave height and the wave propagation velocity increase gradually. The upper limit of RDE's diameter is about 2.80l, which comes from Lagrange quadratic interpolation. The effective diameter range of RDE(taking l as the length unit) in this specific condition is 0.51 ≤ L^* ≤ 2.80.